M+l peak

B By knowing the natural abundances of minor isotopes, it s possible to calculate the relative heights of M+ and M+l peaks. If 33C has a natural abundance of 1.10%, what are the relative heights of theM+ and M+l peaks in the mass spectrum of benzene, C6H6 ... 

The next higher mass peak, M+l, provides information on elemental composition. Table 22-1 lists the natural abundance of several isotopes. For carbon, 98.93% of atoms are, 2C and 1.07% are l3C. Nearly all hydrogen is H, with 0.012% 2H. Applying the factors in Table 22-2 to C Hm, the intensity of the M+l peak should be... 

The mass spectra8 of stereoisomeric inositols contain no peak of the M ion, but only the M+l peak (m/e 181). Fission of water from this ion gives the m/e 163 fragment, which produces the most intense peak with compounds containing H and OH in the cis position at C-l and C-4. Hence, it may be tentatively proposed that 1,4-cis elimination of water takes place, and the ion formed has the structure shown. This ion gives rise to fragment m/e 144, and subsequently to m/e 102. The latter readily eliminates a... 

Whether or not a high-resolution mass spectrometer is available, molecular ion peaks often provide information about the molecular formula. Most elements do not consist of a single isotope, but contain heavier isotopes in varying amounts. These heavier isotopes give rise to small peaks at higher mass numbers than the major M+ molecular ion peak. A peak that is one mass unit heavier than the M+ peak is called the M+l peak two units heavier, the M+2 peak and so on. Table 12-4 gives the isotopic compositions of some common elements, showing how they contribute to M+l and M+2 peaks. 

Ideally, we could use the isotopic compositions in Table 12-4 to determine the entire molecular formula of a compound, by carefully measuring the abundances of the M+, M+l, and M+2 peaks. In practice, however, there are background peaks at every mass number. These background peaks are often similar in intensity to the M+l peak, preventing an accurate measurement of the M+l peak. High-resolution mass spectrometry is much more reliable. 

Look for anything unusual or characteristic about the mass spectrum Does the M+2 peak of the parent ion look larger than the M+l peak It might contain S, Cl, or Br. Is there a large gap and a peak at 127 characteristic of iodine ... 

Fifty per cent of the M-l peak is found at M-2 in the spectra of 3-deu-terioyohimbine (CCCVII, prepared by NaBD4 reduction of 3,4-dehydro-yohimbine perchlorate) and of 3,5,6-trideuterioajmalicine (CCCIX, prepared by NaBD4 reduction of serpentine hydrochloride), so that one-half of the hydrogen lost in the formation of this peak comes from C-3 or, in the case of ajmalicine, from C-3 and C-6. 

The empirical formula of a compound can be established by evaluating the intensities of isotope peaks M + l, M + 2, etc Relative to the parent peak. Isotope peaks arise because of the natural abundance of certain heavier isotopes in some molecules of the sample. Thus a compound containing a single carbon atom will produce anM + 1 peak 1.08% of the intensity of the parent peak because 1.08% of the molecules contain a, 3C atom. If the molecule contains two carbon atoms the M + l peak will be 2.16% of the parent peak and an M + 2 peak due to molect)le containing two, JC atoms will bfc produced of relative intensity 0.01 %. Table 9.10 lists the principal... 

The ionization mechanism in the negative ion mode is a mixture of deprotonation and electron capture. The spectra shown in Fig.3 indicate a fairly even distribution of M - and M-l peaks, although electron capture becomes less important as the source becomes less clean. 

Meperidine HC1 gave a molecular ion of the free base at m/e 2Li7.1563 and there is an intense M-l peak. 

A series of 2,3-disubstituted pyrido[4,3-rf]pyrimidin-4(3//)-ones (154) showed more complicated fragmentation patterns. Here again, however, strong molecular ions were invariably present and m/2e peaks were common, as were M-l peaks. The nature of the group Rx... 

Thus, if the heights of the VT and (M + l) peaks can be measured, it is possible to discriminate between these two compounds that have identical whole-number molecular masses,... 

The mass spectra of S-alkylated thiophenium salts show a characteristic loss of the alkyl group from sulfur, with the result that the spectra appear similar to those of the parent thiophenes. Masses higher than those observed for the parent thiophene rarely corresponded to a molecular ion. In such cases, loss of a hydrogen atom occurred in both methyl and ethyl salts to give an [M — l]" peak, which (when observable) was only of low intensity. Two structures, 4 or 5 have been proposed for this ion. In the case of the S-ethyl salts, further loss of a methyl group may give rise to an [M — 16] peak and the thiapyrylium structure (6) has been advanced for this species. 

Note For the ratio, the M+l peak intensity can be easily estimated in... 

Do not make the mistake of assuming that a calculation of the number of carbon atoms based on the height of the [M+1] peak is set in stone. Any calculation of the number of carbon atoms (or especially the number of oxygen, sulfur or other atoms) is subject to a high degree of error due to the usually small height of the [M+1] (or [M+l]" ) peak. You should always consider the possibility that such calculated numbers could be off by +/- 1. 

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